We develop a method for measuring absolute two-photon absorption cross sections and employ it to determine the of Rhodamine-6G in methanol at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent “z-scan” (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a “thin” (thickness ≪ Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy,optical data storage and optical power limiting.

The x-ray structure factor of water measured under ambient conditions with synchrotron radiation is compared with those predicted on the basis of partial structure factors describing the nuclear positions obtained by neutron diffraction and of different assumptions for the electron distribution. The comparison indicates that a charge of approximately 0.5 e is transferred from each hydrogen atom to the oxygen on the same molecule, implying an effective dipole moment of 2.9 D, in good agreement with theoretical estimates.

An approach for detecting the vibrational spectrum of transient species is demonstrated on the vinyl radical. Photodissociation of carefully chosen precursors at selected photolysis wavelengths produce highly vibrationally excited radicals. Infrared (IR) emission from these radicals is then measured by time-resolvedFourier transform spectroscopy with nanosecond time resolution. All nine vibrational bands of the vinyl radical, generated from four different precursors, are obtained and reported here for the first time.

Recent studies have seriously questioned the use of higher-order Mo/ller–Plesset perturbation theory (MPn) in describing electron correlation in atomic and molecular systems. Here we first reinvestigate with improved numerical techniques previously controversial and disturbing MPn energetic series for Ne, HF, BH, and Conspicuously absent in previous work is research on the convergence of MPnspectroscopic constants, and thus complete MPn (energy, series were then computed for (BH, HF, and through the high orders (MP25, MP21, MP13, MP39 and MP19) within the correlation consistent family of basis sets. A persistent, slowly decaying ringing pattern in the energy series was tracked out to MP155. Finally, new energy series were generated and analyzed through MP167 for and MP39 for Ar and HCl. The MPn energy and property series variously display rapid or slow convergence, monotonic or oscillatory decay, highly erratic or regular behavior, or early or late divergence, all depending on the chemical system or the choice of one-particle basis set. For oscillatory series the spectroscopic constants computed from low-order MPn methods are often more accurate, with respect to the full configuration interaction (FCI) limit, than those computed via high-order MPn theory.

We present a multigrid algorithm for a self-consistent solution of the Kohn–Sham equations in real space. The entire problem is discretized on a real-space mesh with a high-order finite difference representation. The resulting self-consistent equations are solved on a hierarchy of grids of increasing resolution with a nonlinear full approximation scheme, full multigrid algorithm. The self-consistency is effected by updates of the Poisson equation and the exchange-correlation potential at the end of each eigenfunction correction cycle. The algorithm leads to highly efficient solution of the equations, whereby the ground-state electron distribution is obtained in only two or three self-consistency iterations on the finest scale.

The accuracy of standard ab initio wave-function calculations of atomization energies and reactionenthalpies has been assessed by comparing with experimental data for 16 small closed-shell molecules and 13 isogyric reactions. The investigated wave-function models are Hartree–Fock (HF), Møller–Plesset second-order perturbation theory (MP2), coupled-cluster theory with singles and doubles excitations (CCSD) and CCSD with perturbative triple-excitation corrections [CCSD(T)]; the one-electron basis sets used are the correlation-consistent and basis sets with cardinal numbers T, Q, 5, and 6. Results close to the basis-set limit have been obtained by using two-point extrapolations. In agreement with previous studies, it is found that the intrinsic error of the CCSD(T) method is less than chemical accuracy for both atomization energies and reactionenthalpies. The mean and maximum absolute errors of the best CCSD(T) calculations are 0.8 and for the atomization energies and 1.0 and for the reactionenthalpies. Chemical accuracy is obtained already from the extrapolations based on the cc-pCVTZ and cc-pCVQZ basis sets—with mean and maximum absolute errors of 1.7 and for atomization energies and 1.3 and for reactionenthalpies. The intrinsic errors of the Hartree–Fock, MP2, and CCSD wave-function models are significantly larger than for CCSD(T). For CCSD and MP2, the mean absolute errors in the basis set limit are about for the atomization energies and about 10 and respectively, for the reactionenthalpies. For the Hartree–Fock model, the mean absolute errors are 405 and for atomization energies and reactionenthalpies, respectively. Correlation of the core electrons is important in order to obtain accurate results with CCSD(T). Without compromising the accuracy, the core contribution may be calculated with a basis set that has one cardinal number lower than that used for the valence correlation contribution. Basis-set extrapolation should be used for both the core and the valence contributions.

A parallel version of D. Neuhauser’s filter diagonalization algorithm is presented. In contrast to the usual procedure of acting with a set of narrow filter operators on a single or just a few initial vectors, parallelizability is achieved by working with a single, broad filter operator and a correspondingly large number of initial vectors. Apart from the obvious speedup in computation time, there is no need for communication between the processors involved in the computation. Furthermore, because a significantly reduced number of matrix vector multiplications is needed per initial vector, parallel filter diagonalization is numerically more stable than the single processor approach. It is argued that this method is particularly attractive for calculating eigenvectors of the large-scale secular matrices arising in quantum chemistry, especially in dense spectral regions. An application to dense state distributions of a cationic molecular cluster serves as an illustrative example. This is the first time filter diagonalization is used as a tool for ab initio electronic structure calculations.

Two methods of calculating long-range intermolecular potentials are compared for an approximately 3 M aqueous electrolyte solution confined between two charged surfaces. We investigate the ionic density profiles using the charged-sheets method and the corrected three-dimensional (3D) Ewald method at two different system sizes and also compare the Coulomb forces directly. The corrected 3D Ewald method is recommended for the calculation of long-range potentials in systems of this nature because it is less system size dependent than the charged-sheets method and the resultant forces are more consistent with periodic boundaries. However, the charged-sheets method for estimating long-range potentials in Coulombic systems may be useful for certain applications, and the corrected 3D Ewald method also shows some system size dependence.

A new multireference coupled-cluster method which includes double excitations and is based on the complete active space (CAS) multiconfigurational reference wave function is proposed. By partitioning the CAS orbitals into active and nonactive sets a two-component, coupled-clusterwave function involving excitations into orbitals of the different sets was constructed. The first component includes all the CAS excitations and the second component, which has the exponential form, consists of double external and semi-external excitations. The coupled-cluster equations for the energy and for the amplitudes involved in the two components of the wave function were derived and illustrated using a diagrammatic formalism. Several numerical tests were performed, and the results demonstrate a very good performance of the method as compared to the full configuration interaction results.

We computed via first-principles density functional theory calculations (employing both the local density and generalized gradient approximations) the dimensions, bond lengths and angles, binding energy, and HOMO–LUMO gap of the following hypothetical neutral hollow octahedral molecules: (formed by bonding two molecules), and and Each molecule consists of a large hollow framework of six puckered eight-membered rings whose planes are either mutually perpendicular or parallel, so that each molecule possesses only eight- and nine-membered rings. The hydrides have their hydrogen atoms attached only to the two-atom bridging sites on the framework. The oxides have their oxygen atoms occupying exclusively the two-atom bridging sites of the framework alternating with the (B, C, N, Al, Si) atoms exclusively occupying the three-atom bridging sites. We also calculated the infrared spectra of the and the molecules. For the sake of comparison, we also examined the hypothetical octahedral fullerene cuboctohedron (possessing four-, six-, and eight-membered rings) studied by Dunlap and Taylor. The molecules based on carbon would be the most stable; those based on nitrogen would be the least stable, if at all.

In this paper, we report mass spectrometry studies on cluster ions by collision-induced dissociation (CID). The mass-selected cluster ions were dissociated by collision with a crossed nitrogen beam and the fragment ions were mass analyzed in a secondary mass spectrometer. Characterization of the species as cyano-substituted, planar five-membered rings is supported by density functional theory(DFT) calculations. Based on the analysis of the experimental and computational results, cluster structures, energies, and stabilities are discussed.

The infrared spectrum of the weakly bound complex which accompanies the fundamental band of CO in the 4.7 μm region, has been recorded at high resolution using a long-path (≈200 m), low-temperature (≈47 K), absorption cell coupled either with a Fourier transformspectrometer or with a tunable infrared diode laser. A total of 275 transitions, constituting most (>85%) of the observed lines, have been rotationally assigned in terms of 83 discrete quantum states of the complex. The positions of most of these energy levels have been accurately determined (<0.001 cm−1) for both the and 1 vibrational states. The binding energy of the complex, relative to the zero-point level, was determined to be about 30 cm−1. Predicted microwave and millimeter wave frequencies are given for the pure rotational spectrum. The energy level pattern derived here, together with that determined previously for the analogous complex, provides direct and precise information for testing and refining the intermolecular potential energy surface of the carbon monoxide–hydrogen system.

This paper presents ion yields resulting from multiphoton ionization and fragmentation of gaseous toluene in the focus of an 80 fs Ti:sapphire laser beam with a sufficiently small B-integral [Siegman, Lasers (University Science Books, Mill Valley, CA, 1986)]. The peak intensity was varied between and and both linear and circular polarization were used. Over the whole range of intensities studied, only the singly charged parent ion and its fragment, are found. Although the Keldysh adiabaticity parameter equals 0.86 for the saturation intensity of there is no indication of tunneling. The parent ion yield is found to be effectively proportional to the sixth power of the peak intensity. This is shown to be in good agreement with a multiple lowest-order perturbation multiphoton ionization model which takes into account successive channel closing for increasing peak intensities and orders up to 11 inclusive. On the assumption that the excess energy acquired by the toluene cation as a result of the interaction with the electromagnetic field is of the order of the ponderomotive energy for the intensity prevailing at the moment of the ionization, the internal energy distribution of the toluene cations created that is brought about by this multiple-order multiphoton ionization model is calculated. This internal energy distribution is in perfect agreement with the measured yield, if the rate-energy curve for the fragmentation of excited toluene cations as given by Golovin et al. [Sov. J. Chem. Phys. 2, 632 (1985)] is moderately reduced by a factor of 4.5.

We have applied the highly correlated ab initio effective valence shell Hamiltonian method to determine the energy difference between the cyclic and linear isomers of propynlidyne Calculations are also described for the vertical excitation energies,ionization potentials,electron affinities,dipole moments,oscillator strengths, and some harmonic vibrational frequencies, which are all determined using the third order method. Computations at both the experimental and theoretically optimized geometries are used to illustrate the geometrical dependence of the computed properties. The optimized geometry is obtained using a two-configurational reference function describing the two dominant resonance structures. Our third-order vertical excitation energy to the lowest excited state in the cyclic isomer, dipole moments, and ground state isomer conformationalenergy difference are all in good agreement with experiment and with other highly correlated many-body calculations. The computations for higher excited states and for ionization potentials,electron affinities, and oscillator strengths represent the first reports of these quantities. An explanation is provided for persistent theoretical difficulties in computing bending vibrational frequencies of the cyclic isomer.

Binary cluster anions composed of carbon and sulfur atoms have been produced from laser vaporization of a sample mixed with sulfur and carbonpowders in a 20:1 ratio. They were mass-selected and their molecular formula was determined by collision-induced dissociation. The clusters consist of even carbon atoms only and their number of clustering sulfur atoms equals or exceeds that of carbon atoms. Ab initio calculations at the level have been performed for the cluster anions containing two carbon atoms, Geometries of various isomeric structures of the clusters were optimized and their energies were compared to find the most stable isomers. For the singly charged anions, attachment of an additional electron leads to break their sulfur rings, and the isomers with two dangling sulfur atoms and a closed ring have the lowest energy. According to the experimental and theoretical investigations, the two carbon atoms form the bone of the clusters, and ejection of two sulfur atoms is the most competitive dissociation pathway of the cluster anions.

We identify perturbing rovibrational states that are responsible for local J-dependent interactions in the rovibrational manifold of acetylene at ∼11 600 cm−1, observed by infrared-ultraviolet double resonance (IR–UV DR) spectroscopy. These comprise: firstly, the set of vibrational eigenstates that are involved in an avoided crossing with the primary states, as designated in a previous report [Milce and B. J. Orr, J. Chem. Phys. 106, 3592 (1997)]; secondly, a state locally perturbing in the range the adjacent state, which is locally perturbed but with no obvious mixed-in Π-character; and finally, the local perturber of the level at These three vibrational states are now identified and relabeled, according to their most prevalent zero-order normal-mode basis states, as [previously [previously (previously an unidentified perturbing state); and [previously the unidentified local perturber of This analysis is achieved with the aid of the well-established anharmonically coupled polyad model, adapted from a set of generalized quantum numbers for The model has been expanded to include rotational structure, first, in the form of l-resonance off-diagonal elements and, second, in the form of a variety of resonant Coriolis-type interactions. We also predict likely identities for the perturber states involved in unusual odd- symmetry-breaking effects that have been characterized dynamically. It is now postulated that these effects are due to resonant Stark mixing induced by electric fields arising in eithermolecular collisionsor the infrared excitation pulse itself. Coincident ultraviolet probe transitions from doublet levels of opposite parity also contribute to the observed odd-energy transfer ascribed to symmetry breaking.

The low-lying electronic states of and are studied by ab initio calculations with the Stuttgart effective core potentials and corresponding and basis sets. The geometries, vibrational frequencies, and energetic splittings are obtained by the coupled-cluster method including singles and doubles (CCSD) and those including up to the noniterative triples [CCSD(T)] correlation methods with additional frozen core molecular orbitals corresponding to and orbitals. The results for well-studied states are in good agreement with previous experimental results, and therefore our results for other newly studied states are expected to be reliable. The vertical detachment energies of are obtained by the electron excitation equation-of-motion coupled-cluster (EE-EOM-CCSD) method and the average deviation from the experimental results is small without any scaling correction of the obtained values. The effect of the -functions in the basis sets and the noniterative triples in the CCSD(T) method is discussed; the bond lengths are reduced significantly and the vertical detachment energies and ionization potentials are in much better agreement with experiment.

The formation of specific target states in is investigated using phase-locked femtosecond pulse sequences. The pulse sequences generate customized vibrational wave packets whose motion can be interpreted using classical physics. It is shown that, if two vibrational wave packets are initially excited at either end of the vibrational coordinate, changing the initial phase difference between them can have a profound effect on the subsequent dynamics. In particular, the choice of phase differences or has a dramatic effect on the dynamics around the times of the second and fourth order partial revivals. The results are interpreted quantitatively using knowledge of the phase differences between components of fractional revivals evolving from a single wave packet. Finally, we discuss a novel detection technique for monitoring vibrational wave packet dynamics in molecular cations, which combines phase-modulated detection methods borrowed from Rydberg electron wave packet experiments and zero-kinetic energy pulsed-field ionization borrowed from high-resolution spectroscopy.

The ground-state and of are evaluated using the multireference Kramers’ restricted configuration interaction method with relativistic effective core potential and spin–orbit operators. The best computed (estimated) values are 3.11 (3.05) Å, 75 (79) cm−1, and 0.34 (0.38) eV. These results are in accordance with the Raman experimental data of 3.0 Å, 78 cm−1, and for and respectively. The relevant treatment for nondynamic correlations is necessary to obtain reliable spectroscopic constants, although the spin–orbit operators are introduced from the Hartree–Fock stage.

Relativistic coupled cluster studies are performed for the structures, dissociation energies,ionization potentials and electron affinities for Au, and The calculations show that the upward shifts of the ionization potentials and electron affinities of clusters by approximately 2 eV compared to or base on relativistic effects. is predicted to adopt a trigonal planar structure a Jahn–Teller distorted structure 0.1 eV below the linear arrangement, and adopts a linear structure